As one scheme to reduce the cost for configuring a radio communication system, a distributed antenna system (DAS) is implemented. In the distributed antenna system, a signal processing device that processes a transmission signal and a radio device that outputs a radio signal are separated. In the following description, the signal processing device may be referred to as a “digital processing unit”. The radio device may be referred to as a “remote radio unit (RRU)” or a “remote radio head (RRH)”.
A transmission between a digital processing unit and a remote radio unit is implemented by, for example, Radio over Fiber (RoF). A radio frequency signal (RF signal) or an intermediate frequency signal (IF signal) is transmitted via an optical fiber in Radio over Fiber. The configuration in which an intermediate frequency signal is transmitted via an optical fiber may be referred to as IFoF (Intermediate Frequency over Fiber). IFoF is one aspect of RoF. Note that RoF or IFoF are described in, for example, non-patent documents 1-2 below.
FIG. 1 illustrates an example of a Radio over Fiber system. A digital processing unit DU up-converts a data signal to generate a radio frequency signal (hereinafter, RF signal). In this case, the digital processing unit DU converts the RF signal into an optical signal using an E/O (Electrical-to-Optical) circuit 1 and transmits the optical signal to the remote radio unit RRU via an optical fiber link 2. The remote radio unit RRU converts the received optical signal into an electric RF signal using an O/E (Optical-to-Electrical) circuit 3 and amplifies the electric RF signal. The remote radio unit RRU transmits the amplified RF signal via a radio link.
In the signal transmission described above, a signal-to-noise ratio (SNR) or a spurious free dynamic range (SFDR) is deteriorated due to a relative intensity noise (RIN), a shot noise, a thermal noise and so on. For example, when an SFDR of an input signal RFin in the digital processing unit DU is 70 dB-80 dB, an SFDR of an output signal RFout in the remote radio unit RRU may be deteriorated to 45 dB-50 dB.
Relative intensity noise is dominant in deteriorating in SNR/SFDR among the foregoing factors. Thus, it is requested that relative intensity noise be suppressed in order to improve the SNR/SFDR. Relative intensity noise is a parameter indicating a fluctuation in intensity of laser light (that is, intensity noise) and is calculated by dividing an optical intensity noise in a unit frequency by an average optical power. In addition, relative intensity noise is generated by a vibration of a laser cavity, a variation of laser gain medium, and so on. Note that in the Radio over Fiber system illustrated in FIG. 1, relative intensity noise is generated in the E/O circuit 1.
Relative intensity noise depends on an injection current of a laser. Specifically, when the injection current of a laser is small, relative intensity noise is large. Thus, in the Radio over Fiber system illustrated in FIG. 1, if the injection current of a laser in the E/O circuit 1 is increased, SNR/SFDR of an output signal RFout in the remote radio unit RRU may be improved.
Note that related technologies are described in, for example, patent documents 1-6 and a non-patent document 3.    Patent Document 1: Japanese Laid-open Patent Publication No. 3-156379    Patent Document 2: Japanese Laid-open Patent Publication No. 2001-53688    Patent Document 3: Japanese National Publication of International Patent Application No. 2002-503055 (WO99/40696, U.S. Pat. No. 6,819,877)    Patent Document 4: Japanese Laid-open Patent Publication No. 2003-224522    Patent Document 5: US Patent Publication No. 2005/0002469    Patent Document 6: US Patent Publication No. 2008/0057881    Non-Patent Document 1: Charles H. Cox III et. al. “Limits on the Performance of RF-Over-FiberLinks and Their Impact on Device Design”, IEEE Translations on Microwave Theory and Techniques, vol. 54, no 2, pp. 906-920, February 2006.    Non-Patent Document 2: Changyo Han, Seung-Hyun Cho, Hwan Seok Chung, Sang Soo Lee and Jonghyun Lee, “Experimental Comparison of the Multi-IF Carrier Generation Methods in IF-over-Fiber System Using LTE Signals”, MWP 2014, Sapporo, Japan.    Non-Patent Document 3: M. Vasic, O. Garcia, J. A. Oliver, P. Alou, D. Diaz, J. A. Cobos, et al. “High Efficiency Power Amplifier Based on Envelope Elimination and Restoration Technique”, 2010IEEE
As described above, relative intensity noise may be suppressed by increasing the injection current of a laser. The injection current of a laser is proportional to an amplitude of an electric signal to drive the laser (hereinafter, drive signal). That is to say, if an amplitude of a drive signal is increased, a relative intensity noise may be suppressed.
However, an amplitude of a drive signal may greatly change in some modulation schemes of a transmission signal. For example, in a communication system that transmits a signal in OFDM (Orthogonal Frequency Division Multiplexing), M-QAM (M levels Quadrature Amplitude Modulation), W-CDMA (Wideband Code Division Multiple Access) and so on, an amplitude of a drive signal varies greatly and the amplitude of the drive signal may instantaneously become very small. When the amplitude of the drive signal is very small, relative intensity noise is large in an optical transmitter, and SNR/SFDR of a modulated optical signal output from the optical transmitter and an RF signal recovered in an optical receiver is deteriorated.
Note that the problem occurs not only in an optical transmitter used in a Radio over Fiber system but also in an optical transmitter in which an amplitude of an electric signal for driving a laser varies.